It is often useful to provide a voltage to a component on an integrated circuit chip that exceeds a voltage supplied to the chip. Elevated voltages are employed, for example, on DRAM integrated circuits for boosted wordline voltages and negative substrate bias voltages, and for writing and erasing EEPROMS. By generating requisite elevated voltages on the integrated circuit itself, the need for one or more external power supplies is eliminated.
On an integrated circuit, inductors are more difficult to implement than capacitors. Thus, where various voltages are needed on an integrated circuit, it is advantageous to use a capacitive charge pump capable of transforming voltage without the use of inductors.
Two important parameters of charge pump operation are capacity and efficiency. Capacity is a measure of how much current a pump can continuously supply. Capacity is determined in part by the size of a bootstrap capacitor and operating frequency. Efficiency is a measure of how much charge, or current, is wasted during each pump cycle. A typical prior art integrated circuit charge pump is 30-50% efficient. This translates into a loss of 2-3 milliamps of supply current for every milliamp of pump output current.
FIG. 1 shows a conventional single-phase charge pump 10 adapted to receive a first voltage Vcc 12 as an input and provide a second higher voltage as an output. The single phase charge pump includes an inverter 14 having an input 16 adapted to receive an oscillating signal. Also included is a bootstrap capacitor 20 having a driving side 22 and a driven side 24. The inverter is adapted to connect the driving side 22 of capacitor 20 alternately between VCC 12 and ground 30. The driven side 24 of the bootstrap capacitor is operatively connected through a first diode 34 to a source of supply voltage Vcc 12, and through a second diode 36 to a load 38. The load 38 is operatively connected between the second diode 36 and ground 30.
Assuming ideal components, the circuit of FIG. 1 operates as follows: at a first time, an input signal applied at input 16 of the inverter 14 is high, causing the inverter to connect the driving side 22 of capacitor 20 to ground 30. Responsively, current flows through the first diode 34, transporting electrical charge from the source of supply voltage Vcc 12 to the driven side 24 of the bootstrap capacitor 20. As charge accumulates on the driven side 24 of the bootstrap capacitor 20, voltage Vcc 12 develops across the capacitor. At a later time, the signal at the input 16 of the inverter 14 goes low. This connects the driving side 22 of the capacitor 20 to the source of supply voltage Vcc 12. Charge flows into the driving side 22 of the capacitor 20, and the voltage on the driven side 24 of the capacitor rises to 2 Vcc with respect to ground in response. Current flows through the second diode 36 to apply a voltage of 2 Vcc to an input 40 of the load 38. After the voltage on the driving side of the bootstrap capacitor has risen to Vcc, the signal at the input 16 of inverter 14 transitions again causing the voltage on the driving side 22 of capacitor 20 to go to ground. The charge pump cycle is then complete. With repeated cycles, a pulsed voltage of more or less 2 Vcc can be maintained across the load.
As actually constructed, the single-phase charge pump circuit of FIG. 1 is relatively inefficient, and it produces an output voltage that varies significantly with time. Also, for non-ideal components, the output voltage is limited to two times Vcc less at least two diode threshold voltage drops (2 Vt). Accordingly, various improvements have been made to improve charge pump performance as shown for example in U.S. Pat. No. 6,294,948, the disclosure of which is incorporated herein by reference. Circuitry has been developed to bring output voltage up to two times Vcc or higher. It is also known to mutually connect the outputs of two single-phase charge pump circuits, operated out of phase with one another, to reduce ripple and achieve a more constant output voltage. Nonetheless, it is desirable to provide an improved charge pump circuit, and in particular a charge pump circuit which is more efficient than previous designs.